In recent years, many semiconductor devices comprise a plurality of PLL circuits. In such a semiconductor device, when high-speed operation is demanded, the quality of the chip greatly depends on the characteristics of the built-in PLL circuits. V-F characteristics (VCO (Voltage controlled oscillator) control voltage-oscillation frequency characteristics), one of the most important characteristics, of a PLL circuit can be verified by measuring the VCO control voltage. Such a technology for measuring PLL circuits is disclosed in Patent Document 1.
FIG. 16 is a block diagram showing the configuration of a device that measures the transfer characteristics of a phase-locked loop oscillator, described in Patent Document 1. In FIG. 16, a low frequency oscillator (OSC) 101 outputs the frequency having the transfer characteristics that it tries to measure and outputs this output signal to a voltage controlled oscillator (VCO) 102. An output of the voltage controlled oscillator 102 is fed to a phase-locked loop oscillator 103. The phase-locked loop oscillator 103 includes a phase comparator circuit (COMP) 104, a loop filter circuit (FIL) 105, and a voltage controlled oscillator (VCO) 106 that constitute a phase-locked loop, and an output of the voltage controlled oscillator 106 is extracted as an output of the phase-locked loop oscillator.
The transfer characteristics of this phase-locked loop oscillator 103 are measured by feeding the output of the low frequency oscillator 101, i.e., the control signal of the voltage controlled oscillator 102, and an output of the loop filter circuit 105 in the phase-locked loop oscillator 103, i.e., the control signal of the voltage controlled oscillator 106, to a measuring device 107. The measuring device 107 measures the transfer characteristics of the phase-locked loop oscillator 103 by measuring the amplitude and phase ratios of the control signals of the voltage controlled oscillators 102 and 106.
[Patent Document 1]
Japanese Patent Kokai Publication No. JP-A-63-210784